This invention relates to electroluminescent (EL) display panels and more particularly to a light sink layer for use in enhancing the legibility of such a panel under high ambient light conditions.
A typical matrix-addressed EL panel includes a phosphor layer sandwiched between a pair of dielectric layers with a set of transparent indium tin oxide (ITO) electrodes provided over the front dielectric layer thereof and a set of counter aluminum electrodes provided over the rear dielectric layer thereof. When such an operating matrix-addressed EL panel is viewed under high ambient light conditions, a substantial amount of the ambient light entering the front of the EL panel is reflected off the rear aluminum electrodes back to the viewer making it difficult to distinguish between the selectively illuminated pixels and the non-illuminated pixels on the EL panel. Thus, to enhance the contrast between the illuminated and non-illuminated pixels on the EL panel, it has previoulsy been proposed to provide a light absorbing layer immediately behind the phosphor layer for absorbing the incoming ambient light.
The problem with such a light absorbing layer on an EL panel is that heretofore the only materials available for this purpose have been those that have a relatively low electrical specific resistivity. However, because of the multiplexing operation of the matrix-addressed EL panel, wherein the aluminum electrodes are sequentially electrically driven and selected ones of the ITO electrodes are simultaneously electrically driven in accordance with video data such that selected ones of the pixels along the driven aluminum electrode are energized at the same time, it is necessary for the light absorbing layer to be made of a material having a sufficiently high resistivity so that the EL panel can operate with a relatively steep luminance vs. voltage characteristic curve. It is thus seen that it is very important to provide a material for use as a light absorbing layer for a matrix-addressed EL panel which not only can effectively absorb light in the visible range but which also has a relatively high electrical specific resistivity.